redefining tiger subspecies

by Stephen J. O'Brien

Of all the big cats, or perhaps of all endangered species, the tiger may
be both the most charismatic and the most feared. What has been the
evolutionary history framing the tiger into the exquisite predator we admire
today? The rapidly changing field of molecular genetics, particularly advances
in genome-sequence analyses, has provided new tools to reconstruct what
defines a tiger and its origins.

DNA evidence shows that all the thirty-seven living cat species trace back
to a pantherlike predator that lived in Southeast Asia in the late Miocene
Period more than eleven million years ago. The earliest tiger fossils, found in
northern China and Java (Indonesia), date back around two million years.
By the end of the Pliocene and the beginning of the Pleistocene, tigers were
widely distributed in eastern Asia. The coalescence time of modern tiger DNA
(that is, the time elapsed since the merging of lineages backward in time)
occurred around 73,500 years ago, during the late Quaternary, and coincides
with a catastrophic volcanic eruption of Toba in Sumatra, the largest known
explosive volcanic event on Earth. The associated hemispheric volcanic winter
of the Toba supereruption likely persisted for several years and was followed by
a millennium featuring the coldest, driest period in Earth's history.

These events would lead to a near-extinction event for tigers, which, like many
other species of the region, dropped to precariously low numbers. But from these
survivors of the Sunda Shelf cataclysm would arise modern tiger subspecies,
the geographically isolated races of tiger (Amur, Bengal, Indochinese, Malayan,
and Sumatran tigers) that occur today from eastern Russia to Indonesia and
India. The subspecies concept provokes both scientific and political controversy
because living subspecies are considered to be specific units of conservation,
which are protected by international treaties and organizations concerned with
the stewardship of wildlife on the species level. The recognition of subspecies
has particular relevance here because tiger-conservation strategies are inextricably
tied to how subspecies are defined and protected.

Captive populations of living subspecies of tigers have been established
based on the principle that they are genetic representations of their natural
counterparts and thus insurance against extinction in the wild. However,
debates persist over (1) the role of captive tigers in conservation efforts, (2)
whether managed captive populations serve as adequate genetic reservoirs
for the natural populations, and (3) whether the presumptive generic tigers
thought to be decedents of intercross matings between different subspecies
have any conservation value. The most direct way to address the dilemma is by
understanding the genetic ancestry, the extent of genetic admixture, and the
level of genetic diversity of captive tigers in relation to the wild populations.
Traditionally, subspecies were defined by their geographic distribution
combined with morphological traits such as body size, skull traits, coat
color, and striping patterns. Later, several lines of evidence suggested that
the classical subspecies designations were not so reliable. In 2004, my group
and our collaborators published the conclusions of a twenty-year study to
characterize differences among living tiger populations and subspecies using
molecular genetic approaches. We genotyped biological samples from 134
tigers verified as wild-born from a specific geographic locale or descended in
captivity directly from parents of known geographic origins. The results were
striking in providing a variety of subspecies-specific DNA fingerprint markers
unique to different subspecies. Not only were the subspecies shown to be
remarkably distinctive, at least in DNA terms, but the genetic profiles also
indicated that there had been little gene exchange between living subspecies
despite few physical barriers that would keep the tiger populations isolated.
We suspect that behavioral reinforcement--that is, territorial defense as part
of the rigid tiger home-range system--could explain this isolation as it does
for other large cat species.

First recognized as endangered back in 1975, the tiger is vanishing rapidly
from its natural habitat; only an estimated 3,200 remain in the wild as compared with 100,000 a century ago. In contrast to the declining wild tigers, worldwide captive tiger populations are booming: Between
15,000 and 20,000 tigers live in captivity--five to seven times more
than their wild relatives. Only a relatively small portion (less than
1,000 individuals) of the captive-tiger population is managed through
coordinated breeding programs among zoos with the goal of preserving
genetic variability representative of geographic and subspecies groupings
found in the wild. The vast majority of captive tigers are not part
of these managed breeding programs. Most reside in roadside zoos,
breeding farms, makeshift breeding facilities, and circuses and as pets.
Conservation managers generally consider these tigers generic--members
of hybrid subspecies or of unknown origins--and therefore of little
value for conservation matters. This perception may change a bit,
however, because our genetic analyses of captive animals of unknown
provenance revealed that though most were indeed hybrids, between 14
percent and 23 percent were pure subspecies, meaning there are as
many pure validated subspecies in captive settings as there are in
wild habitat. The potential genetic value of tigers housed in these
private facilities means that they may certainly be considered as a source
population for future breeding or even reintroduction programs.

SUMMARY

Modern tiger-genome diversity is traced to a founder event that
occurred 72,000 to 108,000 years ago, coinciding with the Toba
volcano supereruption in Sumatra, Indonesia, which may have reduced
the historical tiger population to a small demographic bottleneck.
Since then, ecological and biogeographical factors have led to the
distinctive population differentiation of at least five surviving
subspecies. Yet a tiger from Sumatra and a tiger from south China have
less genetic distinctiveness between them than a person from Ireland
and a person from India.

Assessment of subspecies ancestry based on DNA, if applied to captive
tigers of uncertain background, would increase by thousands the number
of tigers suitable for conservation management. Considering how
dramatically the tiger population has diminished in wild settings, these
captive populations should probably not be dismissed so quickly.